Here's the FAQs reference for html http://faq.wordpress.com/2006/06/08/allowed-html-tags/
When it comes to using html in comment boxes I have found that this seems to be theme dependent. In some themes html is accepted in the comment boxes and in other themes it's stripped out. Here's an example from the Andreas 09 theme of what's allowed in the comment boxes

tt it shouldn't be theme dependant. Some themes do not include the stuff telling you what is allowed. And some themes use CSS differently. But the stripping process is not carried out at the theme level of abstraction.

To put html into posts it needs to be either *escaped* or the html itself needs to be converted into characters eg &lgt; etc. There are generators that do that or you can do it in your head (if you can remember it) or look up a chart. Comments need the same. I think that's what the guy means. He wants to post code so that it is read. Not processed. I think. :)